Sunday, 20 October 2013

When Size Matters

It is obviously important for a female mammal (or, indeed, any animal) to, so far as she is able, ensure that she mates with the males who are most likely to give her fit and healthy offspring. In the case of monogamous animals, the fact that the male is likely to hang around can be a key part of the equation - monogamy generally occurs where it takes two parents to successfully raise offspring. This is often the case, for example, with birds.

Although clearly there's an advantage, even then, in finding a fit mate, this becomes even more important when the animal isn't monogamous. If you can choose your mate from among several males - none of whom will be around to help you raise the kids, anyway - you might as well pick the best one at the time. There are at least three strategies that can be employed here.

Firstly, you could let them fight it out. Whoever wins is not only bigger and stronger, he's probably also older, which proves that he can survive to that age in an often hostile world. This is a common strategy for many mammal species, and it's perhaps best illustrated in animals that form harems, such as seals, or deer. Here, the males are almost always larger than the females, needing that bulk to fend off younger challengers to their position. They gather a bunch of females around themselves, and drive away weaker males that attempt to mate. (Not, it has to be said, with perfect success, but it's enough that they make a good attempt).

A second strategy is for the males to compete, not physically, but reproductively. In this situation, the females mates with as many males as she possibly can - often in a relatively short time period - and lets their sperm fight it out, inside her reproductive tract, for the honour of fertilising her eggs. In these animals, the males are not necessarily larger than the females, but they do tend to have unusually large testicles for their size, so that can produce a greater quantity of sperm than their rivals, and thus have a better chance of becoming fathers.

Finally, males could try and impress their mates by just looking sexy. This, again, is competition of a sort, and there's no reason that you can't mix this with the first approach. Or even the second, if the female shows any discrimination at all.

Male mammals try to look sexy, in most cases, by growing completely pointless things just to show off. A male stag has huge antlers so that he can say "I am so strong, and such a good forager, that I can afford to waste my hard-earned calories growing these things." Of course, if you can also use whatever you're growing to win fights with other males, so much the better. But you don't have to, and, again, it's something we see a lot of in birds, in the form of extravagant plumage.

But how do you show off if your partner can't see you?

This is the question for, among other animals, golden moles. Golden moles are not at all related to the sort of moles you find in your garden in the northern hemisphere. They live only in southern Africa, and are actually closer to elephants than they are to "true" moles. (Which is to say, not very much). Crucially, they are almost totally blind, with a solid sheet of hairy skin over tiny eyes that never open. They can probably tell light from dark, because knowing that there's light in your underground tunnel tells you that something either has, or is about to, get in and eat you. But that's all; they can't make out shapes.

We know that they're fairly solitary animals, and they aren't forming harems, so its unlikely the males are fighting it out. Their testicles aren't especially large, so it's unlikely there's much sperm competition going on (though it can't be ruled out). Presumably, then, females are choosing their mates, and picking the ones they think are hunkiest. But, when you can't even see your partner, how do you pick the right one?

A recent study, led by Tarryn Retief of the University of Pretoria, made a detailed examination of the anatomy and bodily proportions of Hottentot golden moles (Amblysoma hottentotus) to seek out the answer.

We should, though, try to avoid being too anthropocentric in our thinking here. As social animals, there's a lot more to mate selection in humans than mere physical attraction, but it is true that when we do try to measure physical attractiveness alone, it's mainly sight that we rely on. This doesn't have to be true for other animals. True, northern hemisphere, moles, for instance, seem to find potential partners by scent, and golden moles also seem to have a fairly good sense of smell, so that may be a factor. Sound is also important to them, as one might expect for an animal that lives in total darkness, and fertile females probably use sound to tell the males where they are, and that they're interested.

But, perhaps that's not the whole of the story. If not, we'd expect to find something in their physique that, like a deer's antlers, is larger and more impressive than it needs to be, and that varies significantly between individuals - giving the females something to choose between. The researchers captured a number of golden moles, of both sexes, from a golf course (where they're as much of a nuisance as you'd probably expect), and measured them in detail, seeing how different features varied relative to the overall size of the animal.

The males were slightly larger than females, suggesting that some physical conflict might be going on. This is, after all, how solitary mole-rats compete for mates, alongside sperm competition. But it's much less in golden moles, and likely not a major factor. There was, however, one feature, found in the males only, that did seem to vary quite a lot, and that got larger than you would expect as the males got bigger. That is, larger and healthier males seemed to be diverting energy into making this particular body part even bigger than it would otherwise have been, compared with the rest of their bodies.

Is this what female golden moles seek out as a sign of physical fitness in their partners? What, to borrow a human phrase, they find particularly sexy in a male? Quite possibly.

I refer, of course, to size of their penis.

Perhaps I should pause here to point out that, human preconceptions aside, this really doesn't matter much to most mammals. There is some evidence it's also true of bats, perhaps because, while there's nothing wrong with their eyesight, they do often breed in caves, where vision is useless. In general, though, as male animals get larger, their genitalia change roughly in proportion, or even stay the same size. After all, the females aren't getting any bigger.

Indeed, its interesting to note that, while we're hardly at the top of the league among mammals as a whole, compared to other primates, male human genitalia are really quite large. Consider gorillas, for example. A male silverback gorilla is obviously quite a lot bigger than a human (or even a female gorilla), yet their penis is quite a lot smaller than ours. And that's in absolute terms, not just relative to their overall size.

In humans, it seems plausible that this arose earlier in our evolutionary history than clothing did, making this a desirable trait that females sought out as a proxy for the overall physical fitness of males. (This works both ways, incidentally - human mammary glands are also much more prominent than those of other primates, even when they aren't nursing).

Still, since the female golden moles can't actually see anything, it can't work the same way for them. By the time they find out, the mating has already started. We don't know, from this study alone, quite how it does work, or even for sure that it does. But there are at least a couple of possibilities mentioned in the paper.

One is that there is some sort of subtle sexual competition going on here. That is, since the size of the relevant bits of the female turns out to be fairly constant between individuals, perhaps there is an advantage to males in getting their sperm further up the reproductive tract than their rivals. The other possibility, though, is that the females really do just prefer their males that way, and have some means of ensuring they only become pregnant by their preferred partners.

Indeed, there is evidence that, unlike many other mammals, male golden moles can't force the females to ovulate at the time of mating. Many mammals, such as cats and mice, achieve this using spikes on the end of their penis that dig into the vaginal wall and induce the female to ovulate. (Ouch). Golden moles, it seems, like humans, don't have such spines, so the female does potentially have the opportunity to stop mating before things get too far. We don't know if she actually does this, or if she has some other method at her disposal, but it doesn't contradict the facts.

When we talk about stag antlers or the like, we sometimes say that "size matters" to the female.